Petroleum, natural gas and sulfur industry processing plants have for a number of years utilized a variety of processes for removing hydrogen sulfide from sour gas streams. In general, the gas streams are brought into contact with a circulating liquid alkali absorption solution in a contact zone such as an absorber column, and in successive order the H.sub.2 S is absorbed and converted to an alkali hydrosulfide, thereafter oxidized with a regenerative oxidizing agent to form elemental sulfur, and finally such sulfur is collected for further use. Examples of such processes are the well-known Ferrox, Stretford and Takahax processes, each of which utilize specialized absorption solutions.
While many of the absorptive-regenerative H.sub.2 S removal processes presently available are efficient in removing H.sub.2 S from sour gas streams, workers in the art have found that in many instances solid sulfur tends to build up within plant equipment to an undesirable extent. For example, in some Stretford plants sulfur has been observed to preferentially deposit on the lower packing section of the absorber such as conventional glitch grid packings. In many instances the deposited samples consist of sulfur with alternate layers of gray-to-black sludge identified as vanadium scales. As can be appreciated, such sulfur depositions are extremely troublesome and can accumulate to the point where continued operation of the plant is unfeasible. When this occurs, it is generally necessary to shut down the entire plant, clean these packings by pouring cleaning solution from the top of the absorber through the bottom of the absorber. Alternatively, the packings can be removed from the absorber, cleaned and subsequently reinstalled in the absorber. Apart from the fact that this procedures are costly in and of themselves, the fact that the plant must be shut down during this cleaning period presents obvious difficulties, especially in cases where continual operation is essential. In order to obviate the latter problem, same operators have opted to construct two separate absorbers so that they can be used alternately to permit clean-up of a down absorber without the necessity of complete plant shutdown. Although this expedient ameliorates the problem of undesirable plant shutdown, it will be readily apparent that it is extremely objectionable from a cost standpoint.
It has also been known to send the untreated gas to the flare during the clean-up of the down absorber. This method is again objectionable from a cost standpoint where fuel gas is the product of the H.sub.2 S removal process, since the product fuel gas is burned during the clean-up period. This method also causes undesirably the emission of large quantities of sulfur oxides from burning untreated gas and requires a variance from the local air regulation agencies. The agencies have become increasingly reluctant to issue variances.
U.S. Pat. No. 3,975,508, assigned to J. F. Pritchard and Company, proposes a method for controlling depositions of sulfur within plant equipment by adding ammonia to either the sour gas stream or directly to the absorption solution. This method is not commercially viable, since ammonia is added during the process, the amount of ammonia contained in the treated gas would exceed the level tolerated in the industry since ammonia is added during the process. Also, ammonia added to the feed gas will cause an increase in nitric and nitrous oxide emissions when combusted in standard fired equipment.
Japanese Patent Application Number J51,005,265, assigned to Nippon Steel, proposes a process for removing hydrogen sulfide from gases by conversion of hydrogen sulfide to soluble sulfur compounds using a scrubbing solution containing redox catalyst and an alkaline substance. The problem of sulfur deposit removal is not mentioned in the abstract of the document.
Japanese Patent Application Number J49,077,898, assigned to Nippon Steel, proposes a process for preparing sodium polysulfide by placing waste solution containing carbonate and sulfide with hydrogen sulfide and sodium hydroxide. The problem of sulfur deposit removal is not mentioned in the abstract of the document.
Thus, there remains a need for a commercially feasible method for removing undesirable sulfur depositions within plant equipment utilized with absorptive-regenerative H.sub.2 S removal processes which completely eliminates costly shutdowns or loss of product fuel gas.